The present invention relates to a rotary drum dryer adapted for heating and drying stone aggregate useful in the production of asphalt paving composition or the like.
Conventional asphalt production plants include a drum dryer wherein the stone aggregate is heated and dried and then mixed with liquid asphalt. Such dryers typically comprise a rotating drum which is inclined from the horizontal. The virgin aggregate is introduced into the upper end of the drum, and an outlet is provided adjacent the lower end of the drum for withdrawing the heated and dried aggregate. A burner is mounted adjacent one end of the drum so as to create a heated gas stream which moves through the drum to heat the aggregate flowing therethrough.
In a parallel flow dryer, the burner is mounted adjacent the upper end of the drum so that the heated gas stream moves through the drum in a direction parallel to the moving aggregate, while in a counterflow drum dryer, the burner is mounted adjacent the lower end of the drum and the heated gas flows counter to the direction of movement of the aggregate. A dryer of the parallel flow type is disclosed in U.S. Pat. No. 4,332,478 to Binz, and a dryer of the counterflow type is disclosed in U.S. Pat. No. 4,867,572 to Brock et al.
In drum dryers of the described type, the burner flame defines a combustion zone, and a plurality of longitudinally extending flights extend about the circumference of the inside wall of the drum at the combustion zone, so that the flights shield the wall of the drum from the radiant heat of the burner flame and thereby protect the wall from overheating and deterioration. The need for a protective refractory lining is thus usually unnecessary. These flights are typically T-shaped in transverse cross section, and the flights are spaced from the wall so as to pass over the layer of aggregate in the bottom of the drum as the drum rotates. Thus the flights also tend to shield the aggregate from the radiant heat as the aggregate moves through the combustion zone.
As will be apparent, the above described flights are continually exposed to the radiant heat of the burner flame in the combustion zone, and as a result, the flights themselves become overheated and rapidly deteriorate. Further, the fact that the flights overlie and shield the aggregate in the bottom of the drum, tends to lower the heat absorbed by the aggregate.
It is accordingly an object of the present invention to provide a novel flight construction for the combustion zone of a rotary drum dryer of the described type and which serves to not only effectively shield the wall of the drum from the radiant energy, but also permits more of the aggregate to be exposed to the radiant energy to thereby increase the heat absorbed by the aggregate.
It is another object of the present invention to provide a novel flight construction for a rotary drum dryer of the described type and wherein the flights are covered by the aggregate during a substantial portion of their circular path of travel, and so that the aggregate is exposed to more of the radiant heat energy to increase its temperature, and while the flights are exposed to less radiant energy to decrease their temperature and thus reduce their deterioration.